1. Field of the Invention
This invention relates to air-drying polyurethane polymers obtained from drying oil or semi-drying oil fatty amides. The polymers are acid-functional and are especially useful as water-dispersable air-drying coatings.
2. Description of the Prior Art
Water-reducible polymers containing unsaturated fatty acids as binders for air-drying coatings are known in the prior art. Alkyd resins prepared by the reaction of unsaturated fatty acids and polyols are one example of polymers containing unsaturated fatty acids which are widely used as binders in surface coatings because of their good application and performance characteristics. Alkyd resins are, however, known to suffer from certain disadvantages when used in water-reducible coatings. In particular, the ester groups in the alkyd resins can be hydrolyzed or transesterified upon storage in aqueous medium. This hydrolysis or transesterification can lead to polymer chain degradation, and can also result in the cleavage of the unsaturated fatty acid groups. The unsaturated fatty acid groups act as the crosslinking sites in the polymer and are responsible for the development of coating properties, such as chemical resistance, flexibility, hardness and durability. Therefore, the performance properties of coatings obtained from water-reducible alkyds can deteriorate on storage of the aqueous dispersions of these resins.
Polyurethanes as a class of binders for surface coatings are known to have good chemical resistance, adhesion and durability. The properties are due, at least in part, to the stability of the urethane groups in a variety of environments. Thus, water-reducible unsaturated fatty acid substituted polyurethanes have been utilized as film-forming polymers in order to minimize some of the disadvantages of water-reducible alkyds.
U.S. Pat. No. 3,412,054, issued Nov. 19, 1968 to Milligan and Hoy, discloses water-reducible polyurethanes for use in surface coatings. The unsaturated fatty acids substituted polyurethanes disclosed are obtained by reactions of unsaturated fatty acid ester polyols or unsaturated fatty acids and diisocyanates. Air-drying water-reducible polyurethane modified alkyds are disclosed in U.S. Pat. No. 4,116,902 issued Sept. 26, 1978 to Harris and Pollack. The unsaturated fatty acids described in that patent are part of an alkyd chain which is further reacted with diisocyanates. U.S. Pat. No. 4,147,679, issued Apr. 3, 1979, to Shriven and Chan, also discloses water-reducible urethane coating compositions containing unsaturated fatty acids. Unsaturated fatty acid substituted polyester-polyols are used in the preparation of these polyurethane coatings materials. Water-reducible unsaturated fatty acids substituted polyurethanes are also disclosed in U.S. Pat. Nos. 4,268,426 and 4,277,380, issued May 19, 1981 and July 7, 1981, respectively to Williams and Rogemoser. The unsaturated fatty acids are incorporated in these polyurethanes by the use of the ester-polyols, in particular, unsaturated fatty acid esters of pentaerythritol.
Schneider and Gast (Journal of Coating Technology, 1978, 50 (646), 76) have reported the preparation of water-dispersable polyurethanes from adducts of linseed oil and N,N-diethanolamide. However, the reaction of linseed oil and N,N-diethanolamide gave mixtures of monoglyceride and amide of linseed fatty acids.
Although the polyurethane materials of the prior art provide some improvement in hydrolytic stability over the fatty acid alkyds, the poor hydrolytic stability of alkyds due to cleavage of fatty acid moieties has not been completely overcome.
This invention involves the discovery that water-reducible unsaturated fatty acid substituted polyurethanes can be prepared by the reactions of amide-polyols of unsaturated fatty acids, diisocyanates, and acid-functional compounds which also contain two active hydrogen groups, other than acid groups, which are reactive with the diisocyanate. Since the unsaturated fatty acid moieties of the present invention are attached to the polymer backbone via amide groups, these water-reducible polyurethanes display improved hydrolytic stability when compared to the water-reducible polyurethanes and water-reducible alkyds of the prior art. Aqueous solutions of polyurethanes prepared according to this invention have shown excellent stability after storage at 120.degree. F. for four weeks or more.